1. Field of the Invention
The present general inventive concept relates to an apparatus to extract a sync signal from an analog composite video signal, and more specifically, to an apparatus and method of precisely extracting a sync signal even from a distorted analog composite video signal, thereby preventing distortions in output images.
2. Description of the Related Art
In general, an analog broadcast receiver is required to receive analog broadcasts including NTSC, PAL and SECAM. Despite a high level of availability of digital broadcasts, a large number of viewers still use an analog broadcast receiver. For this reason, analog and digital broadcasts are often simulcasted, and even digital broadcast receivers have a receiving end for receiving the analog broadcasts as well.
In the analog broadcast receiver, if an analog broadcast channel is selected, a tuner receives corresponding radio frequency signals and produces video intermediate frequency signals VIF, such as 45.75 MHz. The video intermediate frequency signal VIF is inputted to a video detector circuit, and the video detector circuit detects the VIF signal and demodulates it to a composite video signal, such as a CVBS signal including color, video, burst, and sync signals. The composite video signal is then inputted to a video decoder and decomposed into an RGB signal for reproducing real colors which are displayed on a display device such as a CRT.
In general, the video decoder extracts the sync signal from the CVBS signal through sync falling edge detection and the sync averaging (to be described later). A system clock is generated using a separated sync average.
FIG. 1A and FIG. 1B are diagrams illustrating a conventional method for extracting a sync signal from a composite video signal. Waveforms depicted in FIGS. 1A and 1B correspond to horizontal sync signals based on the NTSC system.
FIG. 1A describes a method for sync falling edge detection. The method includes detecting a point of half an amplitude of a falling edge of the sync signal. Using a point 101 having a 50% amplitude of the falling edge as a starting position of the horizontal sync signal, an already known rising edge and a vertical sync signal are detected, and a system clock is generated thereby.
FIG. 1B describes a method for sync averaging. The method includes finding a part having 50% of a total area of an incoming horizontal sync signal. At first, an amplitude of a falling edge is divided into two parts, and an area in a lower half is obtained. Using a part (or line) 103, that divides the area marked with slant lines into two equal sub-areas, as a starting point of the horizontal sync signal, a system clock is generated.
However, a problem arises because a normal composite video signal is not always inputted to a video decoder as shown in FIG. 1A or FIG. 1B. For instance, the falling edge or a bottom part of the horizontal sync signal is distorted and then inputted to the video decoder.
FIGS. 2A through 2C are diagrams illustrating methods of extracting an abnormal sync signal from an abnormal composite video signal input.
If a waveform of a video signal shown in FIG. 2A is inputted, the extraction of the sync signal by the sync averaging can be accomplished without causing any problem. Unfortunately, however, when the sync signal is extracted by detecting a falling edge thereof, it is very difficult to know precisely where a bottom 203 of a horizontal sync signal is. As a result, a point 207, not a point 205, is read as a 50% amplitude point on the falling edge, and so, a point 211, not a point 209, is regarded as a starting point of the horizontal sync signal.
Similarly, waveforms shown in FIGS. 2B and 2C are generated if a bottom of an incoming horizontal sync signal is distorted. In this case, however, the sync falling edge detection is a suitable method for sync extraction. If the sync averaging method were applied, a point 215 and a point 219, not points 213 and 217, could be regarded as the starting point of the horizontal sync signal.
Therefore, when the horizontal sync signal wiggles and is not rock stable, a system clock thereof is also affected. Consequently, images on a screen are not quite as sharp, and a HPLL (Horizontal Phase Locked Loop) may be deviated, thereby causing distortions in the images.